Radar level gauge systems for measuring the filling level of a product contained in a tank are well-known. A radar level gauge system generally comprises a transmitter for transmitting an electromagnetic signal towards the surface of the product, a receiver for receiving the electromagnetic signal reflected at the surface of the product, and a signal processing device for determining the level of the product in the tank based on the time-of-flight of the transmitted and reflected electromagnetic signal.
Two main types of radar level gauge (RLG) systems are free radiating RLG systems and so-called guided wave radar (GWR) systems. A free radiating RLG system uses a radiating antenna for directing the transmitted electromagnetic signal towards the surface, and a GWR uses a transmission line wave guide that extends into the product in the tank, typically all the way to the bottom of the tank.
Free radiating RLG systems may use different kinds of antennas for directing the electromagnetic transmission signal towards the surface of the product in the tank. One type of antenna that is often advantageous to use for tank gauging is a horn antenna, which flares out from an entrance opening to an exit opening. When a horn antenna is used, the transmission signal may be fed from the transmitter to the antenna using a hollow waveguide, and the cone antenna functions to provide a smooth transition from propagation in the hollow waveguide to free space propagation below the horn antenna.
Due to, for example, restrictions in the possible dimensions of the horn antenna, there may often remain an impedance transition resulting in reflection of the transmitted electromagnetic signal at the exit opening of the horn antenna.
When the filling level in the tank is in proximity of the exit opening of the horn antenna it may therefore be difficult to determine the filling level due to difficulties in resolving the surface echo signal resulting from reflection of the transmitted electromagnetic signal at the surface of the product in the tank.
According to one approach for minimizing or removing undesired reflections in the horn antenna, WO 2005/109575 discloses a horn antenna that includes an inhomogeneity inside the horn at such a distance from the exit opening of the horn antenna that microwaves reflected by the inhomogeneity and microwaves reflected by the horn edge at the exit opening are canceled out through destructive interference.
Although apparently reducing the reflection at the exit opening of the horn antenna, the approach of WO 2005/109575 would appear to result in an increased loss of signal in the antenna, and a resulting reduction in the maximum measuring distance and/or measurement accuracy of the RLG system.